struct ListNode
{
    int val;
    ListNode *next;
    ListNode() : val(0), next(nullptr) {}
    ListNode(int x) : val(x), next(nullptr) {}
    ListNode(int x, ListNode *next) : val(x), next(next) {}
};


class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        if (!head || !head->next) return head;
        ListNode* reverseHead = nullptr;
        ListNode* curNode = head;
        ListNode* tmp;
        while (curNode) {
            tmp = curNode->next;
            curNode->next = reverseHead;
            reverseHead = curNode;
            curNode = tmp;
        }
        return reverseHead;


    }
};